Add docs

Author: antony-frolov

HISTORY.md

@@ -17,6 +17,8 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
   ([#688](https://github.com/python-attrs/cattrs/pull/688))
 - Python 3.9 is no longer supported, as it is end-of-life. Use previous versions on this Python version.
   ([#698](https://github.com/python-attrs/cattrs/pull/698))
+- Apply the attrs converter to the default value before checking if it is equal to the attribute's value, when `omit_if_default` is true and an attrs converter is specified.
+  ([#696](https://github.com/python-attrs/cattrs/pull/696))
 
 ## 25.3.0 (2025-10-07)
 

src/cattrs/gen/__init__.py

@@ -99,6 +99,10 @@ def make_dict_unstructure_fn_from_attrs(
     ..  versionchanged:: 25.2.0
         The `_cattrs_use_alias` parameter takes its value from the given converter
         by default.
+    .. versionchanged:: NEXT
+        When `_cattrs_omit_if_default` is true and the attribute has an attrs converter
+        specified, the converter is applied to the default value before checking if it
+        is equal to the attribute's value.
     """
 
     fn_name = "unstructure_" + cl.__name__
@@ -186,15 +190,20 @@ def make_dict_unstructure_fn_from_attrs(
             c = a.converter
             if c is not None:
                 conv_name = f"__c_conv_{attr_name}"
-                globs[conv_name] = c
-                internal_arg_parts[conv_name] = c
                 if isinstance(c, Converter):
+                    globs[conv_name] = c
+                    internal_arg_parts[conv_name] = c
                     field_name = f"__c_field_{attr_name}"
                     globs[field_name] = a
                     internal_arg_parts[field_name] = a
                     def_str = f"{conv_name}({def_str}, instance, {field_name})"
-                else:
+                elif isinstance(d, Factory):
+                    globs[conv_name] = c
+                    internal_arg_parts[conv_name] = c
                     def_str = f"{conv_name}({def_str})"
+                else:
+                    globs[def_name] = c(d)
+                    internal_arg_parts[def_name] = c(d)
 
             lines.append(f"  if instance.{attr_name} != {def_str}:")
             lines.append(f"    res['{kn}'] = {invoke}")

tests/test_converter.py

@@ -343,7 +343,7 @@ class C:
 @given(simple_typed_classes(defaults="always", allow_nan=False))
 def test_omit_default_with_attrs_converter_roundtrip(cl_and_vals):
     """
-    Omit default with attrs' converter on the converter works.
+    Omit default with an attrs converter works.
     """
     converter = Converter(omit_if_default=True)
     cl, vals, kwargs = cl_and_vals
@@ -352,16 +352,18 @@ def test_omit_default_with_attrs_converter_roundtrip(cl_and_vals):
     class C:
         a1: int = field(default="1", converter=int)
         a2: int = field(default="1", converter=attrs.Converter(int))
+        a3: int = field(factory=lambda: "1", converter=int)
+        a4: int = field(factory=lambda: "1", converter=attrs.Converter(int))
         c: cl = Factory(lambda: cl(*vals, **kwargs))
 
     inst = C()
     unstructured = converter.unstructure(inst)
     assert unstructured == {}
     assert inst == converter.structure(unstructured, C)
 
-    inst = C(0, 0)
+    inst = C(0, 0, 0, 0)
     unstructured = converter.unstructure(inst)
-    assert unstructured == {"a1": 0, "a2": 0}
+    assert unstructured == {"a1": 0, "a2": 0, "a3": 0, "a4": 0}
     assert inst == converter.structure(unstructured, C)